Nonplanar device with thinned lower body portion and method of fabrication

ABSTRACT

A nonplanar semiconductor device having a semiconductor body formed on an insulating layer of a substrate. The semiconductor body has a top surface opposite a bottom surface formed on the insulating layer and a pair of laterally opposite sidewalls wherein the distance between the laterally opposite sidewalls at the top surface is greater than at the bottom surface. A gate dielectric layer is formed on the top surface of the semiconductor body and on the sidewalls of the semiconductor body. A gate electrode is formed on the gate dielectric layer on the top surface and sidewalls of the semiconductor body. A pair of source/drain regions are formed in the semiconductor body on opposite sides of the gate electrode.

This application is a continuation application of co-pending U.S. patent application Ser. No. 13/243,441 filed Sep. 23, 2011, which is a divisional of U.S. patent application Ser. No. 12/954,241 filed Nov. 24, 2010, now issued as U.S. Pat. No. 8,067,818, which is a divisional of U.S. patent application Ser. No. 10/973,228 filed Oct. 25, 2004, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor devices and more particularly to a nonplanar tri-gate transistor having a thinned lower body portion and method of fabrication.

2. Discussion of Related Art

In order to increase the performance of modern integrated circuits, such as microprocessors, silicon on insulator (SOI) transistors have been proposed. Silicon on insulator (SOI) transistors have an advantage in that they can be operated in a fully depleted manner. Fully depleted transistors have an advantage of ideal subthreshold gradients for optimized ON current/OFF current ratios.

An example of a proposed SOI transistor which can be operated in a fully depleted manner is a tri-gate transistor 100, such as illustrated in FIG. 1. Tri-gate transistor 100 includes a silicon body 104 formed on an insulating substrate 102 having a buried oxide layer 103 formed on a monocrystalline silicon substrate 105. A gate dielectric layer 106 is formed on the top and sidewalls of the silicon body 104 as shown in FIG. 1. A gate electrode 108 is formed on the gate dielectric layer and surrounds the body 104 on three sides, essentially providing a transistor 100 having three gate electrodes (G₁, G₂, G₃), one on each of the sidewalls of the silicon body 104 and one on the top surface of the silicon body 104. A source region 110 and a drain region 112 are formed in the silicon body 104 on opposite sides of the gate electrode 108 as shown in FIG. 1.

An advantage of the tri-gate transistor 100 is that it exhibits good short channel effects (SCE). One reason tri-gate transistor 100 achieves good short channel effects is that the nonplanarity of the device places the gate electrode 108 in such a way as to surround the active channel region. That is, in the tri-gate device, the gate electrode 108 is in contact with three sides of the channel region. Unfortunately, the fourth side, the bottom part of the channel is isolated from the gate electrode by the buried oxide layer 103 and thus is not under close gate control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a nonplanar or tri-gate device.

FIGS. 2A and 2B illustrate a tri-gate or nonplanar device with a thinned lower body portion in accordance with the present invention.

FIG. 3A illustrates a nonplanar device having multiple thinned lower body portions.

FIG. 3B is an illustration of a nonplanar device having a thinned lower body portion and including sidewall spacers, source/drain extensions and silicided source/drain regions.

FIGS. 4A-4H illustrate a method of forming a nonplanar device with a thinned lower body portion in accordance with an embodiment of the present invention.

FIGS. 5A-5D illustrate other semiconductor body profiles or shapes.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention is a novel nonplanar device with a thinned lower body portion and a method of fabrication. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. In other instances, well known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the present invention.

Embodiments of the present invention include a nonplanar or tri-gate transistor having a semiconductor body which is wrapped around on three sides by a gate dielectric layer and a gate electrode. In embodiments of the present invention, the bottom portion of the semiconductor body is made thinner than the top portion of the semiconductor body. Making the bottom portion of the semiconductor body thinner than the top portion increases the gate control over the bottom portion of the body resulting in better short channel effects. In an embodiment of the present invention, a semiconductor film is etched into a semiconductor body utilizing a dry etching process which utilizes a first process gas chemistry and a first RF bias. After forming the semiconductor body, the lower portion of the body is thinned utilizing the same etch chemistry and equipment but utilizing a lower RF bias in order to inwardly taper or facet the lower body portion.

FIGS. 2A and 2B illustrate a nonplanar or tri-gate device 200 having a semiconductor body with a thinned lower body portion. FIG. 2A is an overhead/side view of transistor 200 while FIG. 2B is an illustration of a cross-sectional view taken through the gate electrode. Transistor 200 is formed on a substrate 202 and includes a semiconductor body or fin 204. A gate dielectric layer 206 is formed on the top surface 234 and sidewalls 230 and 232 of a semiconductor body 204. A gate electrode 208 is formed on the gate dielectric layer 206 and surrounds the semiconductor body or fin on three sides. A source regions 210 and a drain region 212 are formed in the semiconductor body on opposite sides of the gate electrode 208 as shown in FIG. 2A.

As is readily apparent from FIGS. 2A and 2B, the semiconductor body 204 has a bottom portion 222 which is thinner than the top portion 224. That is, the distance between the sidewalls 230 and 232 is greater at the top surface 234 than at the bottom surface 236. In an embodiment of the present invention, sidewalls 230 and 232 of the top portion 224 are substantially vertical and are spaced a uniform distance apart while the sidewalls 230 and 232 of the bottom portion 222, are faceted or inwardly tapered to reduce the distance between the sidewalls 230 and 232 in the bottom portion. In an embodiment of the present invention, the distance between the sidewalls 230 and 232 near the bottom surface is between ½ to ⅔ the distance between the sidewalls 230 and 232 near the top surface 234. In an embodiment of the present invention, the sidewalls 230 and 232 begin to taper inwardly at approximately the midpoint of the height 238 of the semiconductor body 204 (i.e., sidewalls start tapering inwardly at the midpoint between the top surface 234 and bottom surface 236). In an embodiment of the present invention, the distance between the sidewalls 230 and 232 at the top surface 234 is between 20-30 nanometers while the distance between the sidewalls 230 and 232 near the bottom surface 236 is between 10-15 nanometers. In an embodiment of the present invention, the bottom portion 222 of the semiconductor body 204 is made sufficiently thin so that the gate control of the bottom portion is made similar to the gate control of the top portion. In an embodiment of the present invention, the bottom portion 222 of the semiconductor body 204 is made sufficiently thin relative to the top portion to improve the short channel effects of transistor 200.

Additionally, as illustrated in FIGS. 5A-5D, other semiconductor body profiles or shapes may be utilized to improve the short channel effects (SCE) of the tri-gate or nonplanar transistor 200. For example, as illustrated in FIG. 5A, the semiconductor body 204 can have a pair of sidewalls 230 and 232 which continually taper inward from the top surface 234 to the bottom surface 236. Additionally, in an embodiment of the present invention, as illustrated in FIG. 5B the semiconductor body 204 can have sidewalls 230 and 232 which continually taper inward from the top surface to the bottom surface and reach the bottom surface 236 at a point or substantially at point 502. In yet another embodiment of the present invention as illustrated in FIG. 5C, the semiconductor body 204 can have a pair of sidewalls 230 and 232 which include an upper vertical portion 510 separated by uniform distance, a middle inwardly tapered portion 512 and a lower portion 514 of vertical sidewalls separated by a second distance which is less than the distance separating the top portion sidewalls 510. In yet another embodiment of the present invention, the semiconductor body can have an upper portion 224 where the sidewalls 230 and 232 are faceted or tapered inwardly and a bottom portion 222 where the sidewalls 230 and 232 are vertical or substantially vertical. In each of the example illustrated in FIGS. 5A-5D, the distance between the sidewalls 230 and 232 of semiconductor body 204 on the top surface is greater than the distance between the semiconductor body on the bottom surface. In this way, the gate electrode 208 can have better control of the semiconductor body at the bottom surface and thereby improve the short channel effects of the device.

In an embodiment of the present invention, the tri-gate transistor 200 is formed on an insulating substrate 202 which includes a lower monocrystalline silicon substrate 250 upon which is formed an insulating layer 252, such as a silicon dioxide film. In an embodiment of the present invention, insulating layer 252 is a buried oxide layer of an SOI substrate. The tri-gate transistor 200, however, can be formed on any well known insulating substrate, such as substrates formed from silicon dioxide, nitrides, oxides, and sapphires.

Semiconductor body 204 is formed on insulating layer 252 of insulating substrate 202. Semiconductor body 204 can be formed on any well known material, such as but not limited to silicon (Si), germanium (Ge), silicon germanium (Si_(x)Ge_(y)), gallium arsenide (GaAs), InSb, GaP and GaSb. Semiconductor body 204 can be formed of any well known material which can be reversely altered from an insulating state to a conductive state by applying external electrical controls. Semiconductor body 204 is ideally a single crystalline film when best electrical performance of transistor 200 is desired. For example, semiconductor body 204 is a single crystalline film when transistor 200 is used in higher performance applications, such as high density circuit, such as a microprocessor. Semiconductor body 204, however, can be a polycrystalline film when transistor 200 is used in applications requiring less stringent performance, such as liquid crystal displays. Insulator 252 isolate semiconductor body 204 from the monocrystalline silicon substrate 250. In an embodiment of the present invention, semiconductor body 204 is a single crystalline silicon film.

Gate dielectric layer 206 is formed on and around three sides of semiconductor body 204 as shown in FIGS. 2A and 2B. Gate dielectric layer 206 is formed on or adjacent to sidewall 230, on the top surface 234 of body 204 and on or adjacent to sidewall 232 of body 204 as shown in FIGS. 2A and 2B. Gate dielectric layer 206 can be any well known gate dielectric layer. In an embodiment of the present invention, the gate dielectric layer is a silicon dioxide (SiO₂), silicon oxynitride (SiO_(x)N_(y)) or a silicon nitride (Si₃N₄) dielectric layer. In an embodiment of the present invention, the gate dielectric layer 206 is a silicon oxynitride film formed to a thickness between 5-20 Å. In an embodiment of the present invention, gate dielectric layer 206 is a high k gate dielectric layer, such as a metal oxide dielectric, such as but not limited to tantalum pentaoxide (TaO₅), titanium oxide (TiO₂) and hafnium oxide (HfO). Gate dielectric layer 206 can be other types of high k dielectric layers, such as but not limited to PZT and BST.

Gate electrode 208 is formed on and around gate dielectric layer 206 as shown in FIGS. 2A and 2B. Gate electrode 208 is formed on or adjacent to gate dielectric layer 206 formed on sidewall 230 of semiconductor body 204 is formed on gate dielectric layer 206 formed on the top surface 234 of semiconductor body 204 and is formed adjacent to or on gate dielectric layer 206 formed on sidewall 232 of semiconductor body 204. Gate electrode 208 has a pair of laterally opposite sidewalls 260 and 262 separated by a distance which defines the gate length (Lg) 264 of transistor 200. In an embodiment of the present invention, laterally opposite sidewalls 260 and 262 of gate electrode 208 run in a direction perpendicular to sidewalls 230 and 232 of semiconductor body 204.

Gate electrode 208 can be formed of any suitable gate electrode material. In an embodiment of the present invention, gate electrode 208 comprises a polycrystalline silicon film doped to a concentration density between 1×10¹⁹ atoms/cm³ to 1×10²⁰ atoms/cm³. In an embodiment of the present invention, the gate electrode can be a metal gate electrode, such as but not limited to tungsten, tantalum, titanium and their nitrides. In an embodiment of the present invention, the gate electrode is formed from a material having a midgap workfunction between 4.5 to 4.8 eV. It is to be appreciated that gate electrode 208 need not necessarily be a single material and can be a composite stack of thin films, such as but not limited to polycrystalline silicon/metal electrode or metal/polycrystalline silicon electrode.

Transistor 200 has a source region 210 and a drain region 212. Source region 210 and drain region 212 are formed in semiconductor 204 on opposite sides of gate electrode 208 as shown in FIG. 2A. Source region 210 and drain region 212 are formed to an n type conductivity type when forming a NMOS transistor and are formed to a p type conductivity when forming a PMOS device. In an embodiment of the present invention, source region 210 and drain region 212 have a doping concentration between 1×10¹⁹ atoms/cm³ to 1×10²¹ atoms/cm³. Source region 210 and drain region 212 can be formed of the uniform concentration or can include subregions of different concentrations or dopant profiles, such as tip regions (e.g., source/drain extensions) and contact regions. In an embodiment of the present invention, when transistor 200 is a symmetrical transistor, source region 210 and drain region 212 have the same doping concentration and profile. In an embodiment of the present invention, when transistor 200 is formed as an asymmetrical transistor, then the doping concentration profile of the source region 210 and drain region 212 may vary in order to any particular electrical characteristics as well known in the art. Source region 210 and drain region 212 can be collectively referred to as a pair of source/drain regions.

The portion of semiconductor body 204 located between source region 210 and drain region 212, defines the channel region 270 of transistor 200. The channel region 270 can also be defined as the area of the semiconductor body 204 surrounded by the gate electrode 208. At times however, the source/drain region may extend slightly beneath the gate electrode through, for example, diffusion to define a channel region slightly smaller than the gate electrode length (Lg). In an embodiment of the present invention channel region 270 is intrinsic or undoped monocrystalline silicon. In an embodiment of the present invention, channel region 270 is doped monocrystalline silicon. When channel region 270 is doped it is typically doped to a conductivity level of between 1×10¹⁶ to 1×10¹⁹ atoms/cm³. In an embodiment of the present invention, when the channel region is doped it is typically doped to the opposite conductivity type of the source region 210 and the drain region 212. For example, when the source and drain regions are n type conductivity the channel region would be doped to p type conductivity. Similarly, when the source and drain regions are p type conductivity the channel region would be n type conductivity. In this manner a tri-gate transistor 200 can be formed into either a NMOS transistor or a PMOS transistor respectively. Channel region 270 can be uniformly doped or can be doped non-uniformly or with differing concentrations to provide particular electrical and performance characteristics. For example, channel regions 270 can include well-known “halo” regions, if desired.

By providing a gate dielectric and a gate electrode which surrounds the semiconductor body on three sides, the tri-gate transistor is characterized in having three channels and three gates, one gate and channel (G₁) which extends between the source and drain regions on side 230 of silicon body 204, a second gate and channel (G₂) which extends between the source and drain regions on the top surface of silicon body 204, and a third gate and channel (G₃) which extends between the source and drain regions on the sidewall of silicon body 204. The gate “width” (Gw) of transistor 200 is the sum of the widths of the three channel regions. That is, the gate width of transistor 200 is equal to the length of sidewall 230 of silicon body 204, plus the length of top surface 234 of silicon body of 204, plus the length of sidewall 232 of silicon body 204. Larger “width” transistors can be obtained by using multiple devices coupled together (e.g., multiple silicon bodies 204 surrounded by a single gate electrode 208) as illustrated in FIG. 3A.

Because the channel region 270 is surrounded on three sides by gate electrode 208 and gate dielectric 206, transistor 200 can be operated in a fully depleted manner wherein when transistor 200 is turned “on” the channel region 270 fully depletes thereby providing the advantageous electrical characteristics and performance of a fully depleted transistor. That is, when transistor 200 is turned “ON” a depletion region is formed in channel region 270 along with an inversion layer at the surfaces of region 270 (i.e., an inversion layer is formed on the side surfaces and top surface of the semiconductor body). The inversion layer has the same conductivity type as the source and drain regions and forms a conductive channel between the source and drain regions to allow current to flow therebetween. The depletion region depletes free carriers from beneath the inversion layer. The depletion region extends to the bottom of channel region 270, thus the transistor can be said to be a “fully depleted” transistor. In embodiments of the present invention, the lower portion 222 of the semiconductor body 204 has been thinned relative to the upper portion so that the gate electrode can better control the lower portion of the semiconductor body. By thinning the lower portion, the two sidewall gates G₁ and G₃ can more easily deplete free carriers from beneath the inversion layer formed on the sidewalls of the lower portion of the semiconductor body 204. By thinning the lower portion 222 of semiconductor body 204, the two gates G₁ and G₃ from the sidewall can control the channel region in a manner similar to the way the three gates G₁, G₂ and G₃ control the channel in the upper portion 224 of the semiconductor body 204. Thinning the bottom part of the body or fin not only decreases the thickness of a semiconductor between the two gates, but also decreases the width of that part of the body which is in contact with the buried oxide. These effects combined decrease the short channel effects in the tri-gate device having a thinned lower body portion.

Transistor 200 of the present invention, can be said to be an nonplanar transistor because the inversion layer of channel 270 is formed in both the horizontal and vertical directions in semiconductor body 204. The semiconductor device of the present invention, can also be considered to be a nonplanar device because the electric field from gate electrode 208 is applied from both horizontal (G₂) and vertical sides (G₁ and G₃).

As stated above the gate width of transistor 200 is equal to the sum of the three gate widths created from semiconductor body 204 of transistor 200. In order to fabricate transistors with larger gate widths, transistor 200 can include additional or multiple semiconductor bodies or fins 204 as illustrated in FIG. 3A. Each semiconductor body or fin 204 has a gate dielectric layer 206 formed on its top surface and sidewalls as shown in FIG. 3A. Gate electrode 208 is formed on and adjacent to each gate dielectric layer 206 on each semiconductor body 204. Each semiconductor body 204 includes a source region 210 and drain region 212 formed in the semiconductor body 204 on opposite sides of gate electrode 208 as shown in FIG. 3A. In an embodiment of the present invention, each semiconductor body 208 is formed with same width and height (thickness) as other semiconductor bodies 204. In an embodiment of the present invention, each source region 210 and drain region 212 of the semiconductor bodies 204 are electrically coupled together by semiconductor material used to form semiconductor body 204 to form a source landing pad 310 and a drain landing pad 312 as shown in FIG. 3A. Alternatively, the source regions 210 and drain regions 212 can be coupled together by higher levels of metallization (e.g., metal 1, metal 2, metal 3) used to electrically interconnect various transistors 200 together in the functional circuits. The gate width of transistor 200 as shown in FIG. 3A would be equal to the sum of the gate width created by each of the semiconductor bodies 204. In this way, a nonplanar or tri-gate transistor 200 can be formed with any gate width desired. In an embodiment of the present invention, each of the semiconductor bodies 204 include a bottom portion 222 which is thinner than the top portion 224 as described above.

In an embodiment of the present invention, the source 210 and drain 212 can include a silicon or other semiconductor film 350 formed on and around semiconductor body 204 as shown in FIG. 3B. For example, semiconductor film 350 can be a silicon film or silicon alloy, such as silicon germanium (Si_(x)Ge_(y)). In an embodiment of the present invention, the semiconductor film 350 is a single crystalline silicon film formed of the same conductivity type as a source region 210 and drain region 212. In an embodiment of the present invention, the semiconductor film can be a silicon alloy, such as silicon germanium where silicon comprises approximately 1-99 atomic percent of the alloy. The semiconductor film 350 need not necessarily be a single crystalline semiconductor film and in embodiment can be a polycrystalline film. In an embodiment of the present invention, semiconductor film 350 is formed on the source region 210 and the drain region 212 of semiconductor body 204 to form “raised” source and drain regions. Semiconductor film 350 can be electrically isolated from a gate electrode 208 by a pair of dielectric sidewalls spacers 360, such as silicon nitride or silicon oxide or composites thereof. Sidewall spacers 360 run along laterally opposite sidewalls 260 and 262 of gate electrode 208 as shown in FIG. 3B thereby isolating the semiconductor film 350 from the gate electrode 208. In an embodiment of the present invention, sidewall spacer 360 have a thickness of between 20-200 Å. By adding a silicon or semiconductor film of the source and drain regions 210 and 212 of the semiconductor body and forming “raised” source and drain regions, the thickness of the source and drain regions is increased thereby reducing the source/drain contact resistance to transistor 200 improving its electrical characteristics and performance.

In an embodiment of the present invention, a silicide film 370, such as but not limited to titanium silicide, nickel silicide, cobalt silicide is formed on the source region 210 and drain region 212. In an embodiment of the present invention, silicide 370 is formed on silicon film 350 on semiconductor body 204 as shown in FIG. 3B. Silicide film 370, however, can be formed directly onto silicon body 204, if desired. Dielectric spacers 360 enables silicide 370 to be formed on semiconductor body 204 or silicon film 250 in a self-aligned process (i.e., a salicide process).

In an embodiment of the present invention, if desired, the silicon film 350 and/or the silicide film 370 can also be formed on the top of gate electrode 208 when gate electrode 208 is a silicon or silicon germanium film. The formation of silicon film 350 and silicide film 370 on the gate electrode 208 reduces the contact resistance of the gate electrode thereby improving the electrical performance of transistor 200.

FIGS. 4A-4H illustrate a method of forming a nonplanar transistor having a thinned lower body portion. The fabrication of the transistor begins with substrate 402. A silicon or semiconductor film 408 is formed on substrate 402 as shown in FIG. 4A. In an embodiment of the present invention, the substrate 402 is an insulating substrate, such as shown in FIG. 4A. In an embodiment of the present invention, insulating substrate 402 includes a lower monocrystalline silicon substrate 404 and a top insulating layer 406, such as a silicon dioxide film or silicon nitride film. Insulating layer 406 isolates semiconductor film 408 from substrate 404, and in embodiment is formed to a thickness between 200-2000 Å. Insulating layer 406 is sometimes referred to as a “buried oxide” layer. When a silicon or semiconductor film 408 is formed on an insulating substrate 402, a silicon or semiconductor on insulating (SOI) substrate is created.

Although semiconductor film 408 is ideally a silicon film, in other embodiments it can be other types of semiconductor films, such as but not limited to germanium (Ge), a silicon germanium alloy (Si_(x)Ge_(y)), gallium arsenide (GaAs), InSb, GaP and GaSb. In an embodiment of the present invention, semiconductor film 408 is an intrinsic (i.e., undoped) silicon film. In other embodiments, semiconductor film 408 is doped to a p type or n type conductivity with a concentration level between 1×10¹⁶-1×10¹⁹ atoms/cm³. Semiconductor film 408 can be insitu doped (i.e., doped while it is deposited) or doped after it is formed on substrate 402 by for example ion-implantation. Doping after formation enables both PMOS and NMOS tri-gate devices to be fabricated easily on the same insulating substrate. The doping level of the semiconductor body at this point can be used to set the doping level of the channel region of the device.

Semiconductor film 408 is formed to a thickness which is approximately equal to the height desired for the subsequently formed semiconductor body or bodies of the fabricated tri-gate transistor. In an embodiment of the present invention, semiconductor film 408 has a thickness or height 409 of less than 30 nanometers and ideally less than 20 nanometers. In an embodiment of the present invention, semiconductor film 408 is formed to the thickness approximately equal to the gate “length” desired of the fabricated tri-gate transistor. In an embodiment of the present invention, semiconductor film 408 is formed thicker than desired gate length of the device. In an embodiment of the present invention, semiconductor film 480 is formed to a thickness which will enable the fabricated tri-gate transistor to be operated in a fully depleted manner for its designed gate length (Lg).

Semiconductor film 408 can be formed on insulating substrate 402 in any well-known method. In one method of forming a silicon on insulator substrate, known as the SIMOX technique, oxygen atoms are implanted at a high dose into a single crystalline silicon substrate and then anneal to form the buried oxide 406 within the substrate. The portion of the single crystalline silicon substrate above the buried oxide becomes the silicon film 408. Another technique currently used to form SOI substrates is an epitaxial silicon film transfer technique which is generally referred to as bonded SOI. In this technique a first silicon wafer has a thin oxide grown on its surface that will later serve as the buried oxide 406 in the SOI structure. Next, a high dose hydrogen implant is made into the first silicon wafer to form a high stress region below the silicon surface of the first wafer. This first wafer is then flipped over and bonded to the surface of a second silicon wafer. The first wafer is then cleaved along the high stress plain created by the hydrogen implant. This results in a SOI structure with a thin silicon layer on top, the buried oxide underneath all on top of the single crystalline silicon substrate. Well-known smoothing techniques, such as HCl smoothing or chemical mechanical polishing (CMP) can be used to smooth the top surface of semiconductor film 408 to its desired thickness.

At this time, if desired, isolation regions (not shown) can be formed into SOI substrate in order to isolate the various transistors to be formed therein from one another. Isolation regions can be formed by etching away portions of the substrate film 408 surrounding a tri-gate transistor, by for example well-known photolithographic and etching techniques, and then back filling the etched regions with an insulating film, such as SiO₂.

In an embodiment of the present invention, a hard mask material 410 formed on semiconductor film 408 as shown in FIG. 4A. Hard mask material 410 is a material which can provide a hard mask for the etching of the semiconductor film 408. A hard mask material is a material which can retain its profile during etching of the semiconductor film 408. A hard mask material 410 is a material which will not etch or only slightly etch during the etching of semiconductor film 408. In an embodiment of the present invention, the hard mask material is formed of a material such that the etchant used to etch the semiconductor film 408 will etch thin film 408 at least five times faster than the hard mask material and ideally at least ten times faster. In an embodiment of the present invention, when semiconductor film 408 is a silicon film, the hard mask material 410 can be a silicon nitride or silicon oxynitride film. Hard mask material 410 is formed to a thickness sufficient to retain its profile during the entire etch of semiconductor film 408 but not too thick to cause difficulty in its patterning. In an embodiment of the present invention, the hard mask material 410 is formed to a thickness between 3 nanometers to 20 nanometers and ideally to a thickness less than 10 nanometers.

Next, as also shown in FIG. 4A, a photoresist mask 412 is formed on hard mask layer 410. Photoresist mask 412 contains a feature pattern to be transferred into the semiconductor film 408. The photoresist mask 412 can be formed by any well known techniques, such as by blanket depositing a photoresist material by masking, exposing and developing the photoresist film into a photoresist mask 412 having a desired pattern for the semiconductor film 408 to be patterned. Photoresist mask 412 is typically formed of an organic compound. Photoresist mask 412 is formed to a thickness sufficient to retain its profile while patterning the hard mask film 410 but yet is not formed to thick to prevent lithographic patterning into the smallest dimensions (i.e., critical dimensions) possible with photolithography system and process used.

Next, as shown in FIG. 4B, the hard mask material 410 is etched in alignment with photoresist mask 412 to form a hard mask 414 as shown in FIG. 4B. Photoresist mask 412 prevents the underlying portion of hard mask material 410 from becoming etched. In an embodiment of the present invention, the hard mask is etched with an etchant which can etch the hard mask material but does not etch the underlying semiconductor film 208. The hard mask material is etched with an etchant that has almost perfect selectivity to the underlying semiconductor film 208. That is, in an embodiment of the present invention, the hard mask etchant etches the hard mask material at least one hundred times faster than the underlying semiconductor film 208 (i.e., an etchant has a hard mask to semiconductor film selectivity of at least 50:1). When the hard mask material 414 is a silicon nitride or silicon oxynitride film, hard mask material 410 can be etched into a hard mask 414 utilizing a dry etch process, such as a reactive ion etching/ecr plasma etching. In an embodiment of the present invention, a silicon nitride or silicon oxynitride hard mask is reactive ion etched utilizing chemistry comprising CHF₃ and O₂ and Ar/CH₂F₂ and C₄F₈ and Ar and O₂.

Next, as shown in FIG. 4C, after hard mask film 410 has been patterned into a hard mask 414, photoresist mask 412 can be removed by well known techniques. For example, photoresist mask 412 can be removed utilizing a “piranha” clean solution which includes sulfuric acid and hydrogen peroxide. Additionally, residue from photoresist mask 412 can be removed with an O₂ ashing.

Although not required, it is desirable to remove photoresist mask 412 prior to etching semiconductor film 408 so that a polymer film from the photoresist does not form on the sidewalls of the patterned semiconductor film 408. It is desirable to first remove the photoresist mask 412 prior to etching of the semiconductor film 408 because dry etching processes can erode the photoresist mask and cause a polymer film to develop on the sidewalls of the semiconductor body which can be hard to remove and which can detrimentally device performance. By first removing the photoresist film 412 prior to patterning the semiconductor thin film 408, the semiconductor thin film 408 can be patterned and pristine sidewalls maintained.

Next, as shown in FIG. 4D, semiconductor film 408 is etched in alignment with hard mask 414 to form a semiconductor body 416 having a pair of laterally opposite sidewalls 418 and 420. Hard mask 414 prevents the underlying portion of semiconductor film 208 from becoming etched during the etching process. The etch is continued until the underlying insulating substrate is reached. In an embodiment of the present invention, the etch “end points” on the buried oxide layer 406. Semiconductor film 208 etched with an etchant which etches semiconductor 208 without significantly etching hard mask 414. In an embodiment of the present invention, semiconductor film 408 is anisotropically etched so that semiconductor body 416 has nearly vertical sidewalls 418 and 420 formed in alignment with the sidewalls of hard mask 414 thereby providing an almost perfect fidelity with hard mask 414. When hard mask 414 is a silicon nitride or silicon oxynitride hard mask and semiconductor film 408 is a silicon film, silicon film 408 can be etched utilizing a dry etch process comprising HBr/Ar/O₂.

In an embodiment of the present invention, semiconductor body 408 is etched utilizing an electron cyclotron residence (ECR) plasma etcher. In an embodiment of the present invention, an ECR plasma etcher using a chemistry comprising HBr/02 with a pressure between 0.2 to 0.8 pascal and the RF power of approximately 120 watts is used to etch a silicon thin film 408 into a silicon body 416. Such an etch process produces a substantially anisotropic etch to provide substantially vertical sidewalls 418 and 420 as shown in FIG. 4D. Additionally, such an etch has a high selectivity (approximately 20:1) to the buried oxide layer 406 so that the buried oxide layer etches very little and can be used as an etch stop and for end point detection. The ability to end point detect is important to insure that all of the semiconductor film clears from the buried oxide layer because the thickness 409 of the thin film across the wafer may vary and the etch rate of different width semiconductor bodies may also vary. In an embodiment of the present invention, an RF bias of between 100-120 watts is used. The RF bias controls the electron energy in the etch which in turn controls the anisotropic profile of the etch.

Next, as shown in FIG. 4F, the semiconductor body 416 is etched so as the reduce the distance between the sidewalls 418 and 420 in the lower portion of the semiconductor body 416. The etching of a semiconductor body to thin the lower portion of the semiconductor body can be referred to as the “profile” etch. In an embodiment of the present invention, the profile etch is utilized to inwardly taper or form facets 422 and 424 on the sidewalls 418 and 420 as illustrated in FIG. 4E. It is to be appreciated that in other embodiments of the present invention, the profile etch can thin the lower body portion as illustrated in FIGS. 5A-5D. In an embodiment of the present invention, a plasma etch process which produces an isotropic etch is utilized to reduce the distance between the sidewalls in lower portion of the semiconductor body as compared to the upper portion of the semiconductor body. In an embodiment of the present invention, the same plasma etch equipment and etch chemistry is used during the profile etch as is used during the patterning of the semiconductor film 408 except that the RF bias is decreased so that the vertical directionality of the ions is reduced. In an embodiment of the present invention, when semiconductor body 416 is a silicon body, the profile etch can be accomplished utilizing an ECR plasma etcher with a chemistry comprising HBr/O2 and a pressure between 0.2 to 0.8 pascal with an RF bias between 50-70 watts.

Next, as also shown in FIG. 4F, the hard mask 414 is removed from semiconductor body 416 having a thinned lower body portion. In an embodiment of the present invention, when hard mask 414 is a silicon nitride or silicon oxynitride film, a wet chemistry comprising phosphoric acid and Di water can be used to remove the hard mask. In an embodiment of the present invention, the hard mask etch comprises between 80-90% phosphoric acid (by volume) and Di water heated to a temperature between 150-170° C. and ideally to 160° C. is used. Such an etchant will have an almost perfect selectivity between the silicon nitride hard mask 214 and buried oxide layer 406.

Next, if desired, after removing hard mask 414 as illustrated in FIG. 4F, semiconductor body 416 can be exposed to a wet etchant to clean the body 416. In an embodiment of the present invention, a silicon body 416 is exposed to a wet etchant comprising ammonia hydroxide (NH₄OH) to remove any line edge roughness or pitting which may have developed during the patterning of the silicon body 416. In an embodiment of the present invention, a silicon body 416 is exposed for a period of time of between 30 seconds to 2 minutes to an etchant comprising between 0.1-1% of ammonia hydroxide by volume at a temperature between 20-30 degrees Celsius in order to provide a semiconductor body 416 with pristine sidewalls 418 and 420.

Next, as illustrated in FIG. 4G, a gate dielectric layer 430 is formed on sidewalls 418 and 420 and the top surface of semiconductor body 416. The gate dielectric layer can be a deposited dielectric or a grown dielectric. In an embodiment of the present invention, the gate dielectric layer 426 is a silicon oxynitride dielectric film grown by a dry/wet oxidation process. In an embodiment of the present invention, the silicon oxide film is grown to a thickness between 5-15 Å. In an embodiment of the present invention, the gate dielectric layer 430 is a deposited dielectric, such as but not limited to a high dielectric constant film, such as a metal oxide dielectric, such as tantalum pentaoxide (Ta₂O₅), titanium oxide (TiO₂), hafnium oxide, zirconium oxide, and aluminum oxide. Additionally, in an embodiment of the present invention, gate dielectric layer 430 can be other high k dielectric films, such as but limited to PZT and BST. Any well known technique can be utilized to deposit a high k dielectric, such as but not limited to chemical vapor deposition, atomic layer deposition and sputtering.

Next, gate electrode 432 is formed on the gate dielectric layer 430 formed on the top surface of semiconductor body 416 and is formed on or adjacent to the gate dielectric layer 430 formed on or adjacent to sidewalls 418 and 420 as shown in FIG. 4G. The gate electrode 432 has a top surface opposite a bottom surface formed on insulating layer 406 and has a pair of laterally opposite sidewalls 434 and 436 which define the gate length of the device. Gate electrode 432 can be formed by blanket depositing a suitable gate electrode material over the substrate and then patterning the gate electrode material with well known photolithograph and etching techniques to form a gate electrode 432 from the gate electrode material. In an embodiment of the present invention, the gate electrode material comprises polycrystalline silicon. In another embodiment of the present invention, the gate electrode material comprises a polycrystalline silicon germanium alloy. In yet other embodiments of the present invention, the gate electrode material can comprise a metal film, such as but not limited to tungsten, tantalum and their nitrides. In an embodiment of the present invention, the photolithography process used to find the gate electrode 432 utilizes the minimum or smallest dimension lithography process used to fabricate the nonplanar transistor (that is, in an embodiment of the present invention, the gate length (Lg) of the gate electrode 432 has a minimum feature dimension of the transistor defined by photolithography). In an embodiment of the present invention, the gate length is less than or equal to 30 nanometers and ideally less than 20 nanometers. It is to be appreciated that although the gate dielectric layer and gate electrode, as illustrated in FIGS. 4G and 4H, are formed with a “subtractive” process whereby undesired portions are etched away, the gate electrode can be formed with a replacement gate process whereby a sacrificial gate electrode is first formed, an interlayer dielectric formed adjacent thereto, the sacrificial gate electrode then removed to form an opening in which the gate electrode is then formed as is well known in the art.

Next, as shown in FIG. 4H, a source region 440 and a drain region 442 are then formed in the semiconductor body 416 on opposite sides of gate electrode 432. For a PMOS transistor, the semiconductor body are doped to a p type conductivity with a concentration between 1×10²⁰ to 1×10²¹ atoms/cm³. For an NMOS nonplanar transistor, the semiconductor body 416 is doped with n type conductivity to a concentration between 1×10²⁰ to 1×10²¹ atmos/cm³ to form the source/drain regions. In an embodiment of the present invention, the source/drain regions can be formed by ion implantation. In an embodiment of the present invention, the ion implantation occurs in a vertical direction (i.e., a direction perpendicular to the substrate) as shown in FIG. 4H. The gate electrode 432 is a polysilicon gate electrode and can be doped during the ion implantation process. The gate electrode 432 acts as a mask to prevent the ion implantation step from doping the channel region of the nonplanar transistor. Again, the channel region is a portion of the semiconductor body 416 located beneath or surrounded by the gate electrode 432. If the gate electrode 432 is a metal electrode a dielectric hard mask can be used to block the doping during ion implantation process. In other embodiments or other methods, such as solid source diffusion may be used to dope the semiconductor body to form the source and drain regions. In embodiments of the present invention, the source/drain regions may also include subregions, such as source/drain extensions and source/drain contact regions. In such a case, the semiconductor body 416 would be doped on either side of the gate electrode 432 to form the source/drain extensions and then a pair of sidewall spacers such as illustrated in FIG. 3B would be formed along the sidewalls of the gate electrode and a second doping step utilized to form heavily doped source/drain contact region as is well known in the art. Additionally, if desired at this time, additional silicon and/or silicide can be formed onto the semiconductor bodies 416 to form raised source/drain regions and reduce the contact resistance of the device. This completes the fabrication of a nonplanar device having a semiconductor body with a thinned lower portion to improve device performance. 

We claim:
 1. A nonplanar tri-gate transistor comprising: a semiconductor body having: a top surface opposite a bottom surface; a pair of laterally opposite sidewalls extending between the top surface and the bottom surface; an upper body portion adjacent the top surface in which the laterally opposite sidewalls are substantially vertical, and a lower body portion extending from the upper body portion to the bottom surface in which the laterally opposite sidewalls continually taper inward from the upper body portion to the bottom surface such that the distance between the laterally opposite sidewalls at the top surface is greater than at the bottom surface; a gate dielectric layer formed on and in direct contact with the top surface and the sidewalls of the semiconductor body from the top surface to the bottom surface; a gate electrode formed on the gate dielectric layer on the top surface and sidewalls of the semiconductor body; and a pair of source/drain regions formed in the semiconductor body on opposite sides of the gate electrode.
 2. The nonplanar tri-gate transistor of claim 1 wherein the distance between the sidewalls at the bottom surface of the semiconductor body is approximately ½ to ⅔ of the distance between the sidewalls at the top surface of the semiconductor body.
 3. The nonplanar tri-gate transistor of claim 1 wherein the distance between the sidewalls at the bottom portion of the semiconductor body is made sufficiently small so as to improve the short channel effects of the transistor.
 4. The nonplanar tri-gate transistor of claim 1 wherein the distance between the laterally opposite sidewalls at the top surface of the semiconductor body is approximately 20-30 nm.
 5. The nonplanar tri-gate transistor of claim 1 wherein the distance between the laterally opposite sidewalls near the bottom portion of the semiconductor body is approximately 10-15 nm.
 6. The nonplanar tri-gate transistor of claim 1 wherein the pair of laterally opposite sidewalls include a pair of faces continually extending from the upper body portion to the bottom surface.
 7. The nonplanar tri-gate transistor of claim 1 wherein the semiconductor body comprises silicon.
 8. The nonplanar tri-gate transistor of claim 1 wherein the distance between the sidewalls at the bottom surface of the semiconductor body is approximately 50-66% of the distance between the sidewalls at the top surface of the semiconductor body.
 9. The nonplanar tri-gate transistor of claim 1 wherein the composition of the gate dielectric layer formed on the top surface is the same composition as the gate dielectric layer formed on the sidewalls.
 10. The nonplanar tri-gate transistor of claim 1 wherein the gate dielectric layer is a continuous gate dielectric layer formed on and in direct contact with the top surface and the sidewalls of the semiconductor body.
 11. The nonplanar tri-gate transistor of claim 1 wherein the top portion meets the bottom portion at approximately a midpoint of a height of the semiconductor body. 